This invention relates to a composition for gas treatment and processes and systems for making and using the composition. In particular, the invention relates to a high capacity regenerable sorbent for removal of mercury from flue gas and processes and systems for making and using the sorbent.
In December, 2000, the U.S. Environmental Protection Agency (EPA) announced its intention to regulate mercury and other air toxics emissions from coal- and oil-fired power plants with implementation as early as November, 2007 (Johnson, J., xe2x80x9cPower Plants to Limit Mercury,xe2x80x9d Chemical and Engineering News, 2001, p. 18, 79). The pending regulation has created an impetus in the utility industry to find cost-effective solutions to meet the impending mercury emission standards,
Mercury and its compounds are a group of chemicals identified in Title III of the 1990 Clean Air Act (CAA) Amendments as air toxic pollutants. Mercury is of significant environmental concern because of its toxicity, persistence in the environment, and bioaccumulation in the food chain. Mercury is one of the most volatile species of the 189 toxic compounds listed in the Clean Air Act Amendments and is, therefore, released readily into the environment from natural and anthropogenic sources. Because of its physical and chemical properties, mercury can also be transported regionally through various environmental cycles (Mercury Study Report to Congress, xe2x80x9cVolume VIII: An Evaluation of Mercury Control Technologies and Costs,xe2x80x9d U.S. Environmental Protection Agency, EPA452/R-97-010, December, 1997). Atmospheric deposition of mercury is reported to be the primary cause of elevated mercury levels in fish found in water bodies remote from known sources of this heavy metal.
Domestic coal-fired power plants emit a total of about fifty metric tons of mercury into the atmosphere annuallyxe2x80x94approximately thirty-three percent of all mercury emissions from U.S. sources (Mercury Study Report to Congress, xe2x80x9cVolume I: Executive Summary,xe2x80x9d United States Environmental Protection Agency, EPA-452/R-97-010, December, 1997; Midwest Research Institute, xe2x80x9cLocating and Estimating Air Emissions from Sources of Mercury and Mercury Compounds,xe2x80x9d EPA-45/R-93-023, September, 1993). Specially designed emission-control systems may be required to capture these volatile compounds effectively. A coal-fired utility boiler emits several different mercury compounds, primarily elemental mercury, mercuric chloride (HgCl2), and mercuric oxide (HgO)xe2x80x94each in different proportions, depending on the characteristics of the fuel being burned and on the method of combustion. Quantifying the rate and composition of mercury emitted from stationary sources has been the subject of much recent work (e.g., Devito, M. S. et al., xe2x80x9cFlue Gas Hg Measurements from Coal-Fired Boilers Equipped with Wet Scrubbers,xe2x80x9d 92nd Annual Meeting Air and Waste Management Association, St. Louis, Mo., Jun. 21-24, 1999; Laudal, D. L. et al., xe2x80x9cBench and Pilot Scale Evaluation of Mercury Measurement Methods,xe2x80x9d DOE/EPRI/EPA Joint Workshop on Mercury Measurement and Speciation Methods, Research Triangle Park, NC, Jan. 29-30, 1997; Hargrove, O. W. et al., xe2x80x9cEnhanced Control of Mercury by Wet FGD,xe2x80x9d proceedings of First Joint Power and Fuel Systems Contractors Conference, Pittsburgh, Pa., Jul. 9-11, 1996; Agbede, R. O., A. J. Bochan, J. L. Clements; R. P. Khosah, T. J. McManus, xe2x80x9cA Comparative Evaluation of EPA Method 29, the Ontario Hydro Method, and New Impinger Solution Methods for the Capture and Analysis of Mercury Species,xe2x80x9d proceedings of the First Joint Power and Fuel Systems Contractors Conference, Pittsburgh, Pa., Jul. 9-11, 1996). Mercury is found predominantly in the vapor-phase in coal-fired boiler flue gas (Mercury Study Report to Congress, xe2x80x9cVolume VIII: An Evaluation of Mercury Control Technologies and Costs,xe2x80x9d United States Environmental Protection Agency, EPA-452/R-97-010, December, 1997). Mercury can also be bound to fly ash in the flue gas. Currently, there is no available control method that efficiently collects all mercury species present in the flue gas stream. Coal-fired combustion flue gas streams are of particular concern because of their composition that includes trace amounts of acid gases, lip including SO2, NO and NO2, and HCl. These acid gases have been shown to degrade the performance of activated carbon, the most widely available sorbent for mercury control, and other proposed sorbents, and so present a challenge that is addressed by the invention described herein.
Today, only municipal solid waste (MSW) incinerators and medical waste combustors are regulated with respect to mercury emissions, and, until the present, the best available control technology for these incinerators is the injection of activated carbon. Although fairly effective for MSW incinerators, activated carbon is a less appealing solution for coal-fired flue gas streams because of the dramatic difference in mercury concentrations. Regulations for mercury control from municipal and medical waste incinerators specify eighty percent control, or outlet emission levels of fifty micrograms per cubic meter (xcexcg/m3). In coal-fired flue gas streams, typical uncontrolled mercury concentrations are on the order of ten xcexcg/m3. For such low concentrations, projected injection rates for activated carbon to maintain ninety percent control of mercury emissions from coal-fired flue gas streams are highxe2x80x94up to 10,000 pounds or more of activated carbon to remove one pound of mercury, depending on the concentration and speciation of mercury in the flue-gas. The mercury-contaminated carbon becomes part of the ash collected by particulate-control devices and can convert the fly ash from an asset to a liability. This impact on the salability of collected fly ash can double the estimated cost of mercury control for a coal-fired power plant.
At present, the injection of activated carbon is broadly considered the best available control technology for reduction of mercury emissions from coal-fired power plants that do not have wet scrubbers (about seventy-five percent of all plants). Tests of carbon injection, both activated and chemically impregnated, have been reported by several research teams: Miller, S. J., et al., xe2x80x9cLaboratory-Scale Investigation of Sorbents for Mercury Control,xe2x80x9d paper number 94-RA 114A.01, presented at the 87th Annual Air and Waste Management Meeting, Cincinnati, Ohio, Jun. 19-24, 1994; Sjostrom, S., J. et al., xe2x80x9cDemonstration of Dry Carbon-Based Sorbent Injection for Mercury Control in Utility ESPs and Baghouses,xe2x80x9d paper 97-WA72A.07, 90th Annual Meeting of the Air and Waste Management Association, Toronto, Ontario, Canada, Jun. 8-13, 1997; Bustard, C. J. et al., xe2x80x9cSorbent Injection for Flue-gas Mercury Control,xe2x80x9d presented at the 87th Annual Air and Waste Management Meeting, Cincinnati, Ohio, Jun. 19-24, 1994; and Butz, J. R. et al., xe2x80x9cUse of sorbents for Air Toxics Control in a Pilot-Scale COHPAC Baghouse,xe2x80x9d 92nd Annual Meeting Air and Waste Management Association, St. Louis, Mo., Jun. 21-24, 1999. Activated carbon injection ratios for effective mercury control are widely variable and are explained by the dependence of the sorption process on flue gas temperature and compostion, mercury speciation and also on fly ash chemistry.
The effectiveness of carbon injection for mercury control is limited by sorbent capacity and flue-gas interactions with the activated carbon. Flue gases contain several acid gases including sulfur dioxide (SO2) in the range of a few hundred to a few thousand parts per million (ppm); hydrogen chloride (hydrochloric acid, HCl) up to 100 ppm; and nitrogen oxides (e.g., NO2) in the range of 200 to 2,000 ppm. Studies reported by Miller, S. J et al., in xe2x80x9cMercury Sorbent Development for Coal-Fired Boilers,xe2x80x9d presented at Conference on Air Quality: Mercury, Trace Elements, and Particulate Matter, McLean, Va., December 1998, at the University of North Dakota""s Energy and Environmental Research Center (EERC) examined the effects of various acid gas constituents on the sorption capacity of carbon in a full-factorial test matrix. The EERC workers fed elemental mercury through carbon samples and systematically changed the gas composition. They noted a limited impact by SO2, but a dramatic drop in capacity when HCl or NO2 were present with SO2. Similar results were obtained in studies in a mercury test fixture by one of the applicants (Turchi, C., xe2x80x9cNovel Process for Removal and Recovery of Vapor-Phase Mercury,xe2x80x9d Final Report for Phase II, DOE Contract DE-AC22-95PC95257, Sep. 29, 2000). Thus, the instability of background art sorbents in an acidic flue gas environment adversely affects the utility of activated carbon sorbents and other sorbents having this limitation.
The cost to implement activated carbon mercury control systems has been estimated by the Department of Energy (DOE), EPA, and several researchers. Chang, R. et al., in xe2x80x9cMercury Emission Control Technologies,xe2x80x9d Power Engineering, November, 1995, pp. 51-56, state that with operating and amortized capital costs, carbon injection will cost between $14,000 and $38,000 per pound of mercury removed, which equates to over $4 million per year for a 250-megawatt (MW) power plant.
EPA estimated similar costs for a 975-MW power plant (Mercury Study Report to Congress, xe2x80x9cVolume VIII: An Evaluation of Mercury Control Technologies and Costs,xe2x80x9d U.S. Environmental Protection Agency, EPA452/R-97-010, December, 1997). In their model, four mercury control scenarios were considered to achieve ninety percent reduction in mercury emissions for a plant with an existing ESP. The scenarios were: (1) activated carbon injection; (2) spray cooling and activated carbon injection; (3) spray cooling, activated carbon injection with added fabric filter collection device; and (4) added activated carbon filter bed. The most economical control option employed spray cooling with carbon injection, resulting in a specific cost of $14,000 per pound mercury removed.
The development of more efficient sorbents that can function in the presence of acids would greatly reduce the cost of this mercury control approach by decreasing the amount of sorbent injected, downsizing sorbent injection equipment, and reducing costs for handling and disposing of spent sorbent.
The potential limitations of carbon-based sorbents cited above have led to research into other possible mercury sorbents. Research has demonstrated that noble-metal-impregnated alumina will remove elemental mercury and mercuric chloride from air streams. The sorbent can be thermally regenerated and the desorbed mercury captured in a condenser or oxidizing wet scrubber. Initial cost estimates looked attractive compared with the cost of disposable carbon sorbents (Turchi et al., xe2x80x9cRemoval of Mercury from Coal Combustion Flue-Gas Using Regenerable Sorbents,xe2x80x9d 92nd Annual Meeting Air and Waste Management Association, St. Louis, Mo., Jun. 21-24, 1999). In 1998 and 1999, work at coal-combustion facilities in Pennsylvania and New Jersey demonstrated that the sorbent can function in flue-gas but at lower efficiency than was seen in the earlier laboratory tests. Subsequent lab work has suggested that acid-gas attack on the sorbent will reduce its effectiveness, as is the case for all background art sorbents. There is also some indication of flue-gas interactions similar to those witnessed by the EERC group. Research is continuing to determine whether the detrimental effects are temporary or permanent.
Fixed beds of zeolites and carbons have been proposed for a variety of mercury-control applications, but none has been developed specifically for control of mercury in coal flue-gas. Products in this class include Lurgi GmbH""s (Frankfurt, Germany) Medisorbon and Calgon Carbon Corporation""s (Pittsburgh, Pa.) HGR. Medisorbon is a sulfur-impregnated zeolite and costs xcx9c$17/lb. As with most sulfur-impregnated materials, Medisorbon loses sulfur when heated above 200xc2x0 F., due to the vapor pressure of sulfur.
The background art is characterized by U.S. Pat. Nos. 3,194,629; 3,873,581; 4,069,140; 4,094,777; 4,101,631; 4,233,274; 4,474,896; 4,721,582; 4,814,152; 4,843,102; 4,843,102; 4,877,515; 4,902,662; 4,911,825; 4,962,276; 4,985,389; 5,080,799; 5,120,515; 5,141,724; 5,173,286; 5,245,106, 5,248,488; 5,409,522; 5,695,726; and 5,846,434; the disclosures of which patents are incorporated by reference as if fully set forth herein.
Dreibelbis et al. in U.S. Pat. No. 3,194,629 discloses a method for removing mercury from gases. The invention is limited in that it relies on impregnation of activated carbon with elemental sulfur. At col. 1, lines 65-67, the reference teaches that xe2x80x9csulfur on alumina was a much poorer adsorbent for mercury vapor than sulfur on activated carbon.xe2x80x9d
Fitzpatrick et al. in U.S. Pat. No. 3,873,581 discloses a process for reducing the level of contaminating mercury in aqueous solutions. The invention is limited in that the process is applied to aqueous solutions and not to gases and it relies on treating an adsorbent with a mercury-reactive factor. Disclosed absorbents are titania, alumina, silica, ferric oxide, stannic oxide, magnesium oxide, kaolin, carbon, calcium sulfate, activated charcoal, activated carbon, activated alumina, activated clay or diatomaceous earth. The adsorbents, the mercury reactant factors and the methods of making and using the sorbents differ from those disclosed herein.
Wunderlich in U.S. Pat. No. 4,069,140 discloses a method for removing arsenic or selenium from a synthetic hydrocarbonaeous fluid by use of a contaminant-removing material. The contaminant-removing material comprises a carrier material and an active material. Carrier materials are selected from the group consisting of silica, alumina, magnesia, zirconia, thoria, zinc oxide, chromium oxide, clay, kieselguhr, filler""s earth, pumice, bauxite and combinations thereof. The active material is selected from the group consisting of iron, cobalt, nickel, at least one oxide of those metals, at least one sulfide of those metals, and combinations thereof. The invention is limited in that teaches sorbent components, methods of sorbent preparation (e.g., involving a calcination step) and methods of sorbent use (e.g., contacting the fluid with the contaminant-removing material in a reducing atmosphere) that differ from those disclosed herein.
Sugier et al. in U.S. Pat. No. 4,094,777 disclose a process for removing mercury from a gas or liquid. This invention is limited in that it requires impregnation of a support only with copper and silver, although other metals can be present, for example iron. Moreover, the supports taught by the reference are limited to silica, alumina, silica-alumina, silicates, aluminates and silico-aluminates. The reference also teaches that incorporation of both metal(s) and pore-forming materials during production of the supports is necessary. Only relatively large absorption masses are envisioned, e.g., alumina balls. Because only large absorption masses are taught, only a fixed bed reactor is taught for contacting the gas with the absorption masses, as would be appropriate for natural gas or electrolytic hydrogen decontamination, which are the only disclosed uses of the compositions and process.
Ambrosini et al. in U.S. Pat. No. 4,101,631 discloses a process for selective absorption of mercury from a gas stream. This invention is limited in that it involves loading a natural or synthetic, three-dimensional, crystalline zeolitic aluminosilicate (zeolite molecular sieve) with elemental sulfur before the zeolite molecular sieve is contacted with the gas stream. Metal sulfides are not present in the zeolite molecular sieve when it is contacted with the gas stream. The use of pellets in absorption beds is disclosed.
Allgulin in U.S. Pat. No. 4,233,274 discloses a method for extracting and recovering mercury from a gas. The invention is limited in that it requires that the gas be contacted with a solution containing mercury(II) ions and ions with the ability to form soluble complexes with such ions.
Chao in U.S. Pat. No. 4,474,896 discloses adsorbent compositions for the adsorption of mercury from gaseous and liquid streams. The invention is limited in that the absorbent compositions are required to contain polysulfide species, while sulfide species may optionally also be present. In the Chao reference, disclosed support materials are limited to carbons, activated carbons, ion-exchange resins, diatomaceous earths, metal oxides, silicates, aluminas, and aluminosilicates, with the most preferred support materials being ion-exchange resins and crystalline aluminosilicate zeolites that undergo a high level of ion-exchange. The open boxworks structures of the preferred zeolite aluminosilicates of the Chao invention differ significantly from the layered structures of the phyllosilicates incorporated into the sorbents disclosed herein. For example, the Chao reference teaches that aluminosilicates of his invention are unstable in the presence of acids (at col. 5, lines 57-68), which is not the case for the phyllosilicates of the invention disclosed herein. In the Chao reference, disclosed metal cations appropriate for ion-exchange or impregnation into the support material are limited to the metal cations of antimony, arsenic, bismuth, cadmium, cobalt, copper, gold, indium, iron, iridium, lead, manganese, molybdenum, mercury, nickel, platinum, silver, tin, tungsten, titanium, vanadium, zinc, zirconium and mixtures thereof. Due to the instability of zeolite support materials in acidic solutions, the Chao reference teaches the exclusion of acidic salt solutions as sources of the metal ions and specifies carboxylic acids, nitrates and sulfates (at col. 5, line 68 and col. 6, lines 1-3). Because polysulfides are a required element of the Chao compositions, disclosed composition production methods include use of a sulfane, heating sulfur and a sulfide-containing support material. The only forms of adsorbent compositions disclosed are {fraction (1/16)}-inch pellets. It is also important to note that the Chao application was to hydrocarbon gas streams, a non-acidic, reducing environment which is not analogous to a flue gas environment.
Nelson in U.S. Pat. No. 4,721,582 discloses a toxic gas absorbent and processes for making the same. The invention is limited to a composition comprising water-laden, exfoliated vermiculite that is coated with pulverulent magnesium oxide.
Yan in U.S. Pat. No. 4,814,152 discloses a composition and process for removing mercury vapor. The composition comprises a solid support that is limited to a carbonaceous support such as activated carbon and activated coke and refractory oxides such as silicas, aluminas, aluminosilicates, e.g., zeolites. The solid support is impregnated with elemental sulfur.
Audeh in U.S. Pat. No. 4,834,953 discloses a process for removing residual mercury from treated natural gas. The process is limited to contacting the gas first with an aqueous polysulfide solution and then with a soluble cobalt salt on a non-reactive carrier material such as alumina, calcium sulfate or a silica.
Horton in U.S. Pat. No. 4,843,102 discloses a process for removal of mercury from gases with an anion exchange resin. The invention is limited in that the anion exchange resin is saturated with a polysulfide solution.
Audeh in U.S. Pat. No. 4,877,515 discloses the use of polysulfide treated molecular sieves (zeolites) to remove mercury from liquefied hydrocarbons. The invention is limited in that the molecular sieve must be pretreated with an alkali polysulfide.
Toulhoat et al. in U.S. Pat. No. 4,902,662 disclose processes for preparing and regenerating a copper-containing, mercury-collecting mass. The mass is made by combining a solid inorganic carrier, a polysulfide and a copper compound. Appropriate solid inorganic carriers are limited to coal, active carbon, coke, silica, silica carbide, silica gel, natural or synthetic silicates, clays, diatomaceous earths, fuller""s earths, kaolin, bauxite, a refractory inorganic oxide such as alumina, titanium oxide, zirconia, magnesia, silica-aluminas, silica-magnesias and silica-zirconias, alumina-boron oxide mixtures, aluminates, silico-aluminates, alumino-silicate crystalline zeolites, synthetic or natural, for example mordenites, faujasites, offretites, erionites, ferrierites, ZSM5 and ZSM11 zeolites, mazzites, and cements.
Roussel et al. in U.S. Pat. No. 4,911,825 disclose a process for elimination of mercury and possibly arsenic in hydrocarbons. The invention requires that a mixture of the hydrocarbon and hydrogen be contacted with a catalyst containing at least one metal from the group consisting of iron, cobalt, nickel and palladium followed by (or mixed with) a capture mass including sulfur or a metal sulfide. The catalyst is preferably deposited on a support chosen from a group limited to alumina, silica-aluminas, silica, zeolites, active carbon, clays and alumina cements.
Yan in U.S. Pat. No. 4,962,276 discloses a process for removing mercury from water or hydrocarbon condensate using a stripping gas. The invention is limited to the use of a polysulfide scrubbing solution for removing the mercury from the stripping gas.
Audeh in U.S. Pat. No. 4,985,389 discloses polysulfide-treated molecular sieves and the use thereof to remove mercury from liquefied hydrocarbons. The molecular sieves are limited to calcined zeolites.
Yan in U.S. Pat. No. 5,080,799 discloses a method for mercury removal from wastewater by regenerative adsorption. The method requires contacting an aqueous stream with an adsorbent composition which includes a metal compound capable of forming an amalgam and/or a sulfide with mercury impregnated into a calcined support. Appropriate metals are limited to bismuth, copper, iron, gold, silver, tin, zinc and palladium and their mixtures. Appropriate supports are limited to those having high surface areas such as Al2O3, SiO2, SiO2/Al2O3, zeolites, clays and active carbon.
Audeh et al. in U.S. Pat. No. 5,120,515 disclose a method for dehydration and removal of residual impurities from gaseous hydrocarbons. The method is limited to replacing an inert protective layer of a pellet with an active compound comprising at least one of copper hydroxide, copper oxide and copper sulfide. Materials for the pellet are limited to alumina, silica-aluminas, molecular sieves, silica gels and combinations thereof.
Audeh et al. in U.S. Pat. No. 5,141,724 disclose a process for removal of mercury from gaseous hydrocarbons. The invention is limited to the use of an in-line mixer which has gas-contacting surfaces of an amalgam-forming metal and a desiccant bed containing pellets of alumina, silica-aluminas, molecular sieves, silica gels, known porous substrates and combinations thereof.
Audeh et al. in U.S. Pat. No. 5,173,286 disclose a process for fixation of elemental mercury present in a spent molecular sieve. The invention is limited to treating the molecular sieve with an aqueous solution containing an alkaline metal salt.
Cameron et al. in U.S. Pat. No. 5,245,106 disclose a method for eliminating mercury or arsenic from a fluid. The process is limited to the incorporation of a copper compound into a solid mineral support, possible calcination of the impregnated support, contact of the impregnated support with elemental sulfur and heat treatment. The solid mineral supports are limited to the group formed by carbon, activated carbon, coke, silica, silicon carbide, silica gel, synthetic or natural silicates, clays, diatomaceous earths, fuller""s earths, kaolin, bauxite, inorganic refractory oxides such as for example alumina, titanium oxide, zirconium, magnesium, alimina-silicas, silica-magnesia and silica-zirconia, mixtures of alumina and boron oxide, the aluminates, silico-aluminates, the crystalline, synthetic or natural zeolitic alumino-silicates, for example the mordenites, faujasites, offretites, erionites, ferrierites, zeolites, ZSM5 and ZSM11, the mazzites and the cements.
Yan in U.S. Pat. No. 5,248,488 discloses a method for removing mercury from natural gas. The method is limited to contacting the natural gas with a sorbent material such as silica, alumina, silica-alumina or activated carbon having deposited on the surfaces thereof an active form of elemental sulfur or sulfur-containing material.
Durham et al. in U.S. Pat. No. 5,409,522 disclose a mercury removal apparatus and method. The invention is limited to the use of a noble metal sorbent.
Lerner in U.S. Pat. No. 5,695,726 discloses a process for removal of mercury and cadmium and their compounds from incinerator flue gas. The invention is limited to contacting a gas containing HCl with a dry alkaline material and a sorbent followed by solids separation. The following sorbents that have an affinity for mercuric chloride are disclosed: activated carbon, fuller""s earth, bentonite and montmorillonite clays.
Seaman et al. in U.S. Pat. No. 5,846,434 disclose an in-situ groundwater remediation process. The process is limited to mobilizing metal oxide colloids with a surfactant and capturing the colloids on a phyllosilicate clay.
The background art is also characterized by non-patent publications. The teachings of these publications are summarized below.
Gash et al., in xe2x80x9cEfficient Recovery of Elemental Mercury from Hg(II)-Contaminated Aqueous Media Using a Redox-Recyclable Ion Exchange Material,xe2x80x9d Environ. Sci. Techno., 1998, pp. 1007-1012, 32(7), American Chemical Society, discloses the use of lithium-intercalated transition metal dichalcogenides as redox-recyclable ion-exchange materials for the extraction of heavy metal ions from water. The reference also discloses a semisynthetic ion-exchange material named thiomont, which is a thioalkylated montmorillonite clay. This reference is limited in that is does not disclose compositions of the type disclosed herein and the compositions that it does disclose can only be used in water treatment.
Dorhout et al., in xe2x80x9cThe Design, Synthesis, and Characterization of Redox-Recyclable Materials for Efficient Extraction of Heavy Metal Ions from Aqueous Waste Streams,xe2x80x9d in New Directions in Materials Synthesis, Winter, C. H., Ed., ACS Symposium Series 727, 1999, pp. 53-68, American Chemical Society, discloses the synthesis and use of lithium-intercalated transition metal disulfides as redox-recyclable materials for the extraction of heavy metals from aqueous waste streams. This reference is limited in that is does not disclose compositions of the type disclosed herein and the compositions that it does disclose can only be used in water treatment.
Brown et al. in xe2x80x9cMercury Measurement and Its Control: What We Know, Have Learned, and Need to Further Investigate.xe2x80x9d in Journal of the Air and Waste Management Association, 1999, pp. 1-97, Air and Waste Management Association, discloses a variety of mercury control technologies in the Air and Waste Management Association""s 29th Annual Critical Review of the state of the art of mercury measurement and control in flue gases produced by the electric utility steam generating industry. It is significant that the description of the state of the art of sorbent injection technologies for mercury capture on pp. 46-80 (which includes a table of proposed fixed-bed sorbents on p. 64) teaches away from the invention disclosed herein and toward activated carbon-based and zeolite-based sorbents.
No individual background art reference or combination of references teach the compositions, processes and systems disclosed herein. In fact, background art references teach away from the elegant solutions proposed herein.
The purpose of the invention is to provide compositions, processes and systems for removal of heavy metals from gas streams, especially those resulting from the combustion of coal which contain the precursors of acid gases such as SO2, NO, NO2, and HCl. One advantage of the invention is that the compositions (sorbents) disclosed herein have a capacity for mercury that greatly exceeds that of the baseline technology, activated carbon. Another advantage is that the disclosed sorbents are unaffected by typical acidic flue gases, which can render background art activated-carbon-based and zeolite-based sorbents virtually useless for this task. Applicants believe that this property is due to the layered structure of the metal sulfide amendments (chalcogenides) used in preferred embodiments of the invention. This layered structure has dimensions such that the polar acid gas molecules are excluded from interlayer sites on the amendment, eliminating the potential for degradation of sorbent performance due to the acid gases. A further advantage is that the strategy of deploying the sorbents into the flue gases as amendments on an inert support maximizes the efficiency and minimizes the costs of the sorbents by exposing only molecularly thin films to the mercury. As a result, all of the sorbent is presented to the flue gas, on a very inexpensive substrate. In addition to having sorption characteristics that are far superior to activated carbon for both elemental and oxidized mercury, the sorbents disclosed herein are less expensive than activated carbon and do not, unlike activated carbon, adversely impact the value of the fly ash, for example, by adversely affecting its use as a concrete additive. Preferred forms of the sorbents disclosed herein ensures that they are xe2x80x9cdrop-inxe2x80x9d replacements for carbon technology and do not require any additional technologies for injection, or collection. The improved capacity and efficiency and the lower costs for the disclosed technology promise to substantially reduce the costs of implementing the EPA""s new emissions controls, benefiting both the utility industry and the U.S. public.
In most flue gas treatment systems, the contact time of a mercury sorbent with a mercury-containing gas is of very brief duration and, therefore, only the surface layers of the sorbent actually perform the sorption. In the invention disclosed herein, the silicate substrate acts an inexpensive support to a thin layer of the polyvalent metal sulfide, ensuring that all of the more expensive metal sulfide engages in the sorption process. While estimated manufactured costs for preferred embodiment of the invention were found to be comparable to the cost of activated carbon, because the mercury sorption capacity of the sorbents disclosed herein is three times the capacity for activated carbon (and much more in the presence of acidic flue gases), the annual operating cost of the disclosed sorbent injection system is expected be no more than one-third the estimated annual operating cost for an activated carbon injection system.
The disclosed compositions are able to adsorb mercury at mass ratios of greater than 1:1 under laboratory conditions. In addition, although the sorbent forms a strong chemical bond with mercury at temperatures typically found with flue gas, the compounds can be thermally regenerated at slightly higher temperatures, allowing for reuse of the sorbent and recovery of mercury for recycling or stabilization.
The disclosed invention is expected to greatly reduce the cost of this mercury control by decreasing the amount of sorbent injected, downsizing sorbent injection equipment, and reducing costs for handling and disposing of spent sorbent. In preferred embodiments, regenerating rather than disposing of spent sorbents is expected to further improve process economics.
The formulation of the sorbents disclosed herein also results in stronger bonding of the mercury to the chemical amendment of the substrate material. The mercury present on used sorbent is thus more difficult to remove, resulting in a final waste form that is more stable and less likely to return the captured mercury to the environment via leaching or other natural processes after disposal.
One object of the invention is to reduce the cost and increase the effectiveness of mercury sorbents and to increase the cost effectiveness of methods and systems for removing mercury from flue gases. Another object of the invention is to prevent contamination of fly ash with activated carbon, thus facilitating its reuse.
In a preferred embodiment, the invention is concerned with a novel mercury sorbent composition. In this embodiment, phyllosilicates having a first layered structure are amended with metal sulfides (e.g., chalcogenides) having a second layered structure, with the open layers of the second layered structure lined with both metal and sulfur ions. While not wishing to be limited by their theory, applicants believe that the layers of the metal sulfides are held together by weak Van der Waals bonds and thus, mercury can enter into these interlayer openings whereas the acid gases are excluded and cannot interfere with the adsorption of mercury. The preferred inter-layer spacing is approximately five nanometers (nm). When the sorbent is used, once mercury has entered the inter-layer spaces, it is retained by interaction with the sulfur-rich environment and amalgamation with the metal ions.
In a preferred embodiment, the invention is concerned with a process for preparing a solid sorbent and product prepared therefrom. The preferred multi-step process includes the steps of obtaining a layered silicate material; performing an ion exchange between the silicate substrate material and a solution containing one or more polyvalent metals from the transition series (e.g., Sn(II), Sn(IV), Fe(II), Fe(III), Ti, Mn, Zn, Mo); washing the impregnated substrate with water; contacting the impregnated and washed substrate with a gas phase or liquid phase source of sulfide; recovering a sulfided substrate (the exchanged polyvalent metal ions precipitate as an insoluble sulfide and become locked in place within the silicate lattice); washing and drying the sulfided substrate; and recovering a high capacity sorbent.
The high capacity sorbent is preferably employed to capture elemental mercury or oxidized mercury species (mercuric chloride) from flue gas and other gases at temperatures from ambient to 350xc2x0 F. A fixed bed may be employed, or the sorbent may be injected directly into the gas stream.
The sorbent may be regenerated by heating to approximately 500xc2x0 F. in a fixed bed or fluidized bed. Preferably, an inert gas (e.g., nitrogen) is flowed through the bed to sweep away the desorbed mercury.
Though not wishing to be bound by their theory, applicants believe that the metal sulfides of the invention disclosed herein act as very efficient sorbents for heavy metals such as mercury due to their planar crystal lattice structure. The crystal lattices of the sorbents of the subject invention are arranged in planar arrays creating open layers, lined with sulfur atoms and/or metal ions. The open nature of these layers permits ready access for mercury atoms and ions, but the spacing of the parallel planes is such that acid gases cannot contact the metal sulfide molecules, and therefore do not impact the performance of the sorbent. The sulfur atoms have a strong affinity for mercury, which becomes tightly bound within the crystal lattice. Furthermore, certain metals such as tin and titanium form amalgams with mercury, further enhancing the binding mechanism. In the case of tin (II) sulfide, for example, the interlayer gaps are lined with alternating rows of tin and sulfide atoms. Advantageously, the mercury thus has the potential to bind to every atom of the silicate amendment.
Preferred embodiments of the disclosed compositions comprise transitional metal dichalcogenides (TMDs) and/or polyvalent metal sulfides (PVMS). These TMD and PVMS compounds have a layered structure with opposing sulfur atoms. The gap formed between the layers create an interplanar space, where heavy metals are highly attracted due to the high density of the sulfur atoms. Further, the interlayer spacing is such that acid gas molecules are excluded from the space, and thus cannot impact the performance of the sorbent. This two-dimensional layered structure creates compounds similar in many ways to graphite. Uptake of metals occurs by insertion of the metal within the two-dimensional layered structure in a phenomenon known as intercalation. Intercalation is a chemical insertion reaction by which atoms (generally metals) can be inserted between the layers of two-dimensional layered compounds without altering the basic structure of the host material. Tin and metals in the first few columns of the transition block of the periodic table are capable of forming these layered structures.
Preferred embodiments of the disclosed compositions also comprise a substrate, preferably a phyllosilicate. In phyllosilicate minerals, rings of tetrahedrons are linked by shared oxygens to other rings in a two dimensional plane that produces a sheet-like structure. Typically, the sheets are then connected to each other by layers of cations. These cation layers arc weakly bonded and often have water molecules and other neutral atoms or molecules trapped between the sheets. The silicon to oxygen ratio is generally 1:2.5 (or 2:5) because only one oxygen is exclusively bonded to the silicon and the other three are half shared (1.5) to other silicons. The symmetry of the members of this group is controlled chiefly by the symmetry of the rings but is usually altered to a lower symmetry by other ions and other layers; but the overlying symmetry of the silicate sheets will usually still be expressed in a higher pseudo-symmetry. The typical crystal habit of phyllosilicates is flat, platy, book-like and most all members display good basal cleavage. Although members tend to be soft, they are remarkably resilient. Phyllosilicates are also generally tolerant of high pressures and temperatures.
Vermiculite (i.e., hydrated laminar magnesium-aluminum-ironsilicate that resembles mica in appearance) is one preferred sorbent substrate, given its ion exchange capacity, commercial availability, and low cost. Vermiculite is the name applied to a group of magnesium aluminum iron silicate minerals, with a variable composition that may be summarized thus:
(Mg, Ca)0.7(Mg, Fe3+, Al)6.0[(Al, Si)8O20](OH)4.8H2O
Flakes of raw vermiculite concentrate are micaceous in appearance and contain interlayers of water in their structure. When the flakes are heated rapidly, or treated with hydrogen peroxide, the flakes expand, or exfoliate, into accordion-like particles. The resulting lightweight material is chemically inert, fire resistant, and odorless. Vermiculite is widely used in lightweight plaster and concrete, providing good thermal insulation. For this reason, the addition of vermiculite to fly ash materials is not likely to affect the properties of concrete made with it, unlike the addition of carbon to fly ash.
Vermiculite is a phyllo-, or layered silicate with a generalized structure similar to that of talc. It has a central, octahedrally coordinated layer of iron and magnesium oxides lying between two inwardly pointing sheets of silica tetrahedra. In vermiculite, iron and magnesium ions substitute for silicon in the tetrahedral layer and the resulting electrical imbalance is neutralized by loosely bound interlayer ions of calcium, magnesium, or more rarely, sodium. The interlayer space also includes two ordered layers of water molecules. The calcium and magnesium ions within the interlayer space can be replaced by other metal ions to give vermiculite a very high ion-exchange capacity. Vermiculite is not described in the literature as an aluminosilicate.
Montmorillonite is another preferred sorbent substrate. Montmorillonite, also known as smectite, bentonite, or Fuller""s Earth, is a clay weathering product of alumino-silicate minerals. These clays typically develop in semi-arid regions from solutions with high concentrations of magnesium ions and can be made synthetically. Montmorillonite is a crypto-crystalline aluminosilicate. Montmorillonite clays are constructed of a single sheet of alumina octahedra, sandwiched between two layers of silica tetrahedra. Substitution of other atoms (Mg2+, Fe2+, or Mn2+) for the aluminum in the octahedral layer or Al3+ substitution for silicon in the tetrahedral layer leads to interlayer charge imbalance, producing one excess negative charge for each substituted atom. Cations intercalate into the interlayer areas to balance the charge. Water molecules are also present in the interlayer areas.
The hydrated interlayer space between the sheets is expansible, that is, the separation between the individual smectite sheets varies depending upon the interlayer cations present. Because the interlayer area is hydrated, cations within the interlayer may easily exchange with cations in an external solution, providing that charge balance is maintained. This leads to very high cation exchange capacities in these materials that may be as high as 80-150 mEq/100 g. The availability of the interlayer areas and the very small particle size lead to these clays having extremely large effective surface areas.
The typical particulate size of montmorillonite is in the range of a few microns diameter, which makes it easy to inject and suspend in a flue gas stream, where it is exposed to mercury. By using a particle size that is similar to the fly ash, thorough mixing of the sorbent material into the gas stream is assured. This in turn minimizes the mass of sorbent that is required to achieve mandated mercury removal levels. Its handling and injection into a flue gas stream is similar to that of activated carbon, done with conventional materials handling equipment and requiring residence times for the sorbent on the order of about one second to achieve superior mercury removal from the flue gas stream. The silica content of the montmorillonite renders it easily collectable in an electrostatic precipitator. And that same silica content also renders the collected fly ash and sorbent mixture readily salable as a pozzolan material, a clear advantage over activated carbon.
Allophane is another preferred sorbent substrate. Allophane is a synthetic amorphous aluminosilicate with a high cation exchange capacity. Thus, both natural and synthetic amorphous aluminosilicate materials are preferred as sorbent substrates.
In a preferred embodiment, sorbent preparation is a multi-step process that includes the exchange of metals and addition of sulfide ions to the phyllosilicate substrate material. Preferably, the first step in the preparation of the sorbent is an ion exchange between the phyllosilicate substrate material and a solution containing one or more of a group of polyvalent metals including tin (both Sn(II) and Sn(IV)), iron (both Fe(II) and Fe(III)), titanium, manganese, zirconium and molybdenum, dissolved as salts, such as the sulfate, chloride or nitrate, or as other soluble forms. Ion exchange is preferably performed by suspending or otherwise contacting the phyllosilicate substrate with the solution containing a metal ion for a period of time sufficient to complete the process. Preferably, the solution is stirred or mixed during this time to facilitate the exchange. When the ion exchange is complete, the phyllosilicate substrate material is preferably separated from the solution and may be briefly washed with water. Separation can be accomplished by any number of means, many of which are well established and generally known to those in the field of process engineering. Examples include settling, filtration, and centrifugation. In a preferred embodiment, the metal solution is reused directly or processed to recover unused metal ions.
In a preferred embodiment, contact between the phyllosilicate substrate and the metal ion solution occurs as a multi-step process in which quantities of substrate and solution are sequentially. contacted with each other in stages. This process is preferably performed in either a co-current or counter-current manner.
In a preferred embodiment, the second step in the preparation of the sorbent is the controlled addition of sulfide ions to the phyllosilicate substrate as described below. This is preferably accomplished by contacting the exchanged phyllosilicate substrate with a gas phase or a liquid phase source of sulfide.
Preferable sources of sulfur or sulfide for gas-phase contacting include hydrogen sulfide. Preferable sources of sulfur or sulfide for liquid-phase contacting include sodium sulfide (Na2S) or a compound containing sulfur with different oxidation states, e.g., calcium polysulfide (CaSx). The preferred oxidation state of sulfur species for liquid-phase contacting is minus two. Another option includes dissolving thiourea in water and contacting that solution with the ion-exchanged phyllosilicate. Preferably, excess hydrogen sulfide generated through the addition of sulfide is trapped in an alkaline solution of sodium hydroxide or calcium sulfide for reuse.
Sulfide addition in the liquid phase is preferably accomplished by the incremental addition of a solution containing a sulfide to a liquid containing the phyllosilicate substrate material impregnated with the exchanged polyvalent metal ions. Preferably, during sulfide addition the pH of the liquid is monitored and the acidic pH of the exchanged phyllosilicate is adjusted by the addition of the alkaline sulfide solution to neutrality. Step-wise addition of the sulfide solution is complete when a desired quantity of sulfide is added or when the desired pH is obtained. The sulfurization step is conducted by adding the sulfur source to the metal-impregnated sorbent solution until the pH of the solution is between 6 and 9, and preferably within 7+/xe2x88x920.5 units. During this process, the exchanged polyvalent metal ions precipitate as an insoluble sulfide and become locked in place within the phyllosilicate lattice. In a preferred embodiment, the amended phyllosilicate is then separated from the solution using a conventional separation technique, e.g., centrifugation, filtration, or flotation, and washed with water (preferably, distilled water). Preferably, the amended phyllosilicate material is then dried using a conventional technique, e.g., electric heat drying, heating in an oven or kiln, vacuum drying, passing a dry, inert gas through the amended phyllosilicate material, or spray drying. Preferably, the amended phyllosilicate material is dried to less than about five percent moisture such that the material is flowable.
In another preferred embodiment, the sorbent disclosed herein is used to absorb elemental mercury or oxidized mercury species such as mercuric chloride from flue gas and other gases at temperatures from ambient to as high as 350xc2x0 F. In a preferred embodiment, the operational temperature range for the sorption process operating at near ambient pressure conditions is about 350xc2x0 F. and less. Sorption processes conducted at higher pressures (e.g., at sixty psi) can be operated at temperatures near 500xc2x0 F. Applicants believe that 350xc2x0 F. is a likely practical upper limit for the sorption process at atmospheric pressure. Sorbent performance degrades with increasing temperature, and improves with decreasing temperature. The practical lower limit is related to the acid dew point of the gas stream, at which the SO2 in the gas begins to condense to form sulfuric acid. This can become a major corrosion issue in ductwork of a power plant. For high sulfur coals, where SO2 levels can easily be a few thousand ppm, the acid dew point can be in the range 250-270xc2x0 F. For low-sulfur coals, the SO2 is typically 400 ppm or less, and the acid dew point may be less than 180xc2x0 F.
Absorption takes place while the sorbent is in contact with the gas. This can occur in a number of conventional process configurations, e.g., injection into the flue gas stream traveling from the economizer to the particulate control equipment or as a fixed bed installed downstream of the particulate control equipment. In a preferred embodiment, the sorbent is contained within a fixed bed in which it is substantially stationary. In this embodiment, contact between the gas and sorbent is achieved as the gas flows through the bed. Applicants believe that a few seconds of contact is adequate. The size of the bed is more typically dependent on how often it is to be changed or regenerated. In a preferred embodiment, an empty bed residence time of less than one second, nominally 0.7 second is provided. Applicants believe that an optimum contact time is 1.5 seconds.
Another process configuration for sorbent use comprises directly injecting and entraining the sorbent into the gas stream. For a coal-fired power plant, sorbent is preferably injected into the gas stream downstream of the boiler and remains in the gas stream until it is removed along with the flyash using an electrostatic precipitator and/or a baghouse. In this configuration, contact is achieved while the sorbent is entrained in the gas and also during the time it is fixed to the separation device. In any configuration, an adequate contact time (preferably at least one or two seconds) is required to ensure proper sorption of the mercury onto or into the sorbent.
In a preferred embodiment, when mercury sorption is complete, the sorbent is stabilized and disposed of using any of a variety of conventional techniques, e.g., landfilled along with the collected fly ash or sold along with the fly ash for use as a.pozzolan additive in concrete. The sorbent may also be regenerated by heating it to about 500xc2x0 F. and maintaining it at that temperature for a time that is sufficient to desorb the mercury from the sorbent. Preferably, sorbent regeneration occurs in a fixed or fluidized bed. During the regeneration step, an inert gas such as nitrogen is preferably flowed through the bed to sweep desorbed mercury away from the sorbent. Preferably, desorbed mercury is captured for reuse or disposal using any of a variety of conventional techniques, e.g., the mercury can be condensed as the liquid element in a chilled condenser, or captured as mercuric oxide in a chemical wet scrubber. In preferred embodiments, desorption from a fixed bed takes about one-half to one times the total sorption time to which the sorbent has been subjected.
The above description is for a preferred operating mode in which the pressures at which mercury is adsorbed onto and into the sorbent and at which it is desorbed are approximately equal. The adsorption and desorption modes are determined primarily by variations in the sorbent temperature. In another preferred operating mode, the temperatures at which adsorption and desorption occur are essentially equal (and may be very high, e.g., at least 700xc2x0 F., can be between 500-1000xc2x0 F. For this case, the adsorption and desorption modes are determined primarily by variations in the operating pressure. This operating mode is referred to as Pressure-Swing-Adsorption (PSA) and is a well-known separation technique. The high-capacity sorbent described herein may be used in either operating mode. In a preferred embodiment, sorbent is used to remove mercury from a gas stream at a gas pressure of sixty pounds per square inch gauge (psig) and a temperature of 500xc2x0 F. In this embodiment, mercury is released from the sorbent when the pressure is reduced by less than ten percent (five psig) while the same sorption temperature is maintained. Laboratory testing at 4 atmospheres (atm) gauge) indicated that a pressure swing of 0.6 atm was sufficient to cause desorption. Applicants expect typical operation to be in the range of 15-80 atm.
In a preferred embodiment, the invention is a sorbent particle comprising: a vermiculite having a plurality of ion-exchange sites; a plurality of polyvalent metal ions exchanged at some of said ion-exchange sites; and a plurality of sulfide ions chemically reacted to at least some of said polyvalent metal ions. In this embodiment, the sorbent particle is essentially devoid of polysulfides, said sorbent particle has a largest dimension of less than about twenty micrometers and said sorbent particle is operative to capture at least ninety percent of the ionic and elemental mercury present in a flue gas containing acid gases (e.g., SO2, NO and NO2, and/or HCl) to which it is exposed.
In another preferred embodiment, the invention is a sorbent particle comprising: a montmorillonite having a plurality of ion-exchange sites; a plurality of polyvalent metal ions exchanged at some of said ion-exchange sites; and a plurality of sulfide ions chemically reacted to at least some of said polyvalent metal ions. In this embodiment, the sorbent particle is essentially devoid of polysulfides, said sorbent particle has a largest dimension of less than about twenty micrometers and said sorbent particle is operative to capture at least some of the ionic and elemental mercury present in a flue gas containing acid gases to which it is exposed.
In yet another preferred embodiment, the invention is a sorbent particle comprising: a cryptocrystalline phyllosilicate having a plurality of ion-exchange sites; a plurality of polyvalent metal ions exchanged at some of said ion-exchange sites; and a plurality of sulfide ions chemically reacted to at least some of said polyvalent metal ions. In this embodiment, the sorbent particle is essentially devoid of polysulfides, said sorbent particle has a largest dimension of less than about twenty micrometers and said sorbent particle is operative to capture at least some of the ionic and elemental mercury present in flue gas to which it is exposed.
In a further preferred embodiment, the invention is a sorbent comprising: a phyllosilicate having a plurality of ion-exchange sites; a plurality of polyvalent metal ions exchanged at some of said ion-exchange sites; and a plurality of sulfide ions chemically reacted to at least some of said polyvalent metal ions. In this embodiment, the sorbent is operative to accomplish sustained removal of the ionic and elemental mercury present in an acidic flue gas to which it is exposed.
In another preferred embodiment, the invention is a sorbent comprising: a non-zeolitic, amorphous aluminosilicate having a plurality of ion-exchange sites; a plurality of polyvalent metal ions exchanged at some of said ion-exchange sites; and a plurality of sulfide ions chemically reacted to at least some of said polyvalent metal ions. In this embodiment, the sorbent is essentially devoid of copper and polysulfides.
In yet another preferred embodiment, the invention is an adsorbent composition for use in the adsorption of ionic and elemental mercury consisting essentially of: a non-zeolitic aluminosilicate support material having cation sites, the material being selected from the class consisting of vermiculites, allophane and montmorillonites; a cation selected from the group consisting of antimony, arsenic, bismuth, cadmium, cobalt, gold, indium, iron, lead, manganese, molybdenum, mercury, nickel, platinum, silver, tin, tungsten, titanium, vanadium, zinc, zirconium and mixtures thereof wherein the cation occupies some of the cation sites; and a sulfide.
In a further preferred embodiment, the invention is an adsorbent composition for use in the adsorption of ionic and elemental mercury consisting essentially of: a phyllosilicate support material having cation sites, the material being selected from the class consisting of vermiculites and montmorillonites; a cation selected from the group consisting of antimony, arsenic, bismuth, cadmium, cobalt, copper, gold, indium, iron, lead, manganese, molybdenum, mercury, nickel, platinum, silver, tin, tungsten, titanium, vanadium, zinc, zirconium and mixtures thereof wherein the cation occupies some of the cation sites; and a sulfide.
In another preferred embodiment, the invention is a sorbent comprising: a silicate having a first layered structure selected from the group consisting of kaolinite, halloysite, montmorillonite, illite, bentonite, chlorite, and vermiculite; impregnated with a metal sulfide having a second layered structure. In this embodiment, the sorbent is essentially free of polysulfides. Preferably, the metal sulfide is selected from the group consisting of polyvalent metal sulfides.
In another preferred embodiment, the invention is a composition of matter consisting of: a hydrated laminar magnesium aluminum ironsilicate having a plurality of ion-exchange sites; a polyvalent metal ion derived from a highly acidic solution exchanged at some of said ion-exchange sites; and a plurality of sulfide ions chemically reacted to some of said polyvalent metal ions.
In yet another preferred embodiment, the invention is a composition of matter consisting essentially of: montmorillonite having a plurality of ion-exchange sites; a polyvalent metal ion exchanged at some of said ion-exchange sites; and a plurality of sulfide ions chemically reacted to said polyvalent metal ions. In this embodiment, each sulfide ion has the formula Sxxe2x88x922 wherein x is 1.
In another preferred embodiment, the invention is a composition of matter made by combining: phyllosilicate substrate material having a plurality of ion-exchange sites at which cations are exchangeable; a plurality of polyvalent metal ions derived from a highly acidic solution that are exchanged at some of said ion-exchange sites; and a plurality of sulfide ions which are chemically reactable with some of said polyvalent metal ions.
In a further preferred embodiment, the invention is a composition made by combining effective amounts of: means for supporting having a first layered structure and a plurality of ion-exchange sites at which cations are exchangeable; a plurality of polyvalent metal ions which are reversibly substituted at some of said ion-exchange sites; and a plurality of sulfide ions which are chemically reacted to some of said polyvalent metal ions to produce a second layered structure having an inter-layer spacing of about five nanometers. In this embodiment, the composition comprises essentially no polysulfide ions and is capable of removing mercury from a gas stream containing trace amounts of acid gases.
In another preferred embodiment, the invention is a composition made by combining effective amounts of: a synthetic montinorillinite having a plurality of ion-exchange sites at which cations are exchangeable; a plurality of polyvalent metal ions in a highly acidic solution which are reversibly substituted at some of said ion-exchange sites; and a plurality of sulfide ions that are other than copper ions which are chemically reacted to some of said polyvalent metal ions. In this embodiment, the composition is essentially devoid of polysulfide ions and is capable of sorbing mercury from a gas.
In another preferred embodiment, the invention is an adsorbent composition for use in the adsorption of elemental mercury consisting essentially of: a support material selected from the class consisting of phyllosilicates; a cation selected from the group consisting of a bivalent tin ion, a tetravalent tin ion, a bivalent iron ion, a trivalent iron ion, a titanium ion, a manganese ion, a zirconium ion, a vanadium ion, a zinc ion, a nickel ion, a bismuth ion, a cobalt ion, and a molybdenum ion; and a sulfide. In this embodiment, the composition is essentially devoid of polysulfides. Preferably, the phyllosilicate is selected from the class consisting of vermiculite and montmorillinite and the at least one cation is selected from the group consisting of copper, cobalt, manganese, nickel and mixtures thereof.
The invention is also a process and apparatus for preparing the sorbent. In a preferred embodiment, the invention is a process for the preparation of sorbent particles for ionic and elemental mercury comprising: (a) reducing the size of a phyllosilicate support material having cation sites, the material being selected from the class consisting of vermiculites and montmorillonites, to a particle having a largest dimension of less than about twenty micrometers; (b) providing the particle of step (a) with at least one cation capable of forming an insoluble sulfide and selected from the group consisting of antimony, arsenic, bismuth, cadmium, cobalt, gold, indium, iron, lead, manganese, molybdenum, mercury, nickel, platinum, silver, tin, tungsten, titanium, vanadium, zinc, zirconium and mixtures thereof; and (c) contacting the cation-containing particle of step (b) with a solution containing a sulfide-forming species and devoid of a polysulfide-forming species to produce a sorbent particle that is operative to capture at least some of the ionic and elemental mercury present in flue gas containing trace amounts of acid gas species to which it is exposed.
In another preferred embodiment, the invention is a process for the preparation of adsorbent compositions for elemental mercury comprising: providing a support material selected from the class consisting of phyllosilicates with at least one cation capable of forming an insoluble sulfide and selected from the group consisting of antimony, arsenic, bismuth, cadmium, cobalt, gold, indium, iron, lead, manganese, molybdenum, mercury, nickel, platinum, silver, tin, tungsten, titanium, vanadium, zinc, zirconium and mixtures thereof; and contacting the cation-containing support material of the foregoing step with a sulfide-forming species and not a polysulfide-forming species.
In another preferred embodiment, the invention is a process for producing a sorbent particle comprising: reducing the size of a phyllosilicate material (e.g., by grinding or other conventional means) to produce a phyllosilicate particle having a largest dimension of less than about twenty micrometers; contacting (e.g., in a first reactor) the phyllosilicate particle with a highly acidic solution containing a plurality of polyvalent metal ions other than copper ions to produce an exchanged phyllosilicate particle; separating the exchanged phyllosilicate particle from the solution; contacting (e.g., in a second reactor) the exchanged phyllosilicate particle with a fluid containing a plurality of sulfide ions and devoid of polysulfide ions to produce an amended phyllosilicate particle; and separating the amended phyllosilicate particle from the fluid to produce a sorbent particle that is operative to capture at least some of the ionic and elemental mercury present in flue gas to which it is exposed.
In a further preferred embodiment, the invention is a process for producing a sorbent particle comprising: reducing the size of a vermiculite material to produce a vermiculite particle having a first layered structure and a largest dimension of less than about twenty micrometers;
contacting the vermiculite particle with a solution containing a plurality of polyvalent metal ions to produce an exchanged vermiculite particle; separating the exchanged vermiculite particle from the solution; contacting the exchanged vermiculite particle with a fluid containing a plurality of sulfide ions and devoid of polysulfide ions to produce an amended vermiculite particle containing an amendment having a second layered structure; and separating the amended vermiculite particle from the fluid to produce a sorbent particle that is operative to capture at least some of the ionic and elemental mercury present in flue gas to which it is exposed.
In yet another preferred embodiment, the invention is a process for producing a sorbent particle comprising: reducing the size of a montmorillonite material to produce a montmorillonite particle having a largest dimension of less than about twenty micrometers; contacting the montmorillonite particle with a solution containing a plurality of polyvalent metal ions to produce an exchanged montmorillonite particle; separating the exchanged montmorillonite particle from the solution; contacting the exchanged montmorillonite particle with a fluid containing a plurality of sulfide ions and devoid of polysulfide ions to produce an amended montmorillonite particle; and separating the amended montmorillonite particle from the fluid to produce a sorbent particle that is operative to capture at least some of the ionic and elemental mercury present in flue gas to which it is exposed.
In another preferred embodiment, the invention is a process for preparing a sorbent composition comprising: contacting a support material with a highly acidic solution containing at least one cation capable of forming an insoluble sulfide other than a copper sulfide;
contacting the cation containing support of the previous step with a species that is capable of forming a sulfide but not a polysulfide.
In another preferred embodiment, the invention is a process for producing a sorbent comprising: contacting a phyllosilicate substrate material with a solution containing a polyvalent metal ion to produce an exchanged phyllosilicate; separating the exchanged phyllosilicate from the solution; contacting the exchanged phyllosilicate with a fluid containing a sulfide ion other than a polysulfide ion to produce an amended phyllosilicate; separating the amended phyllosilicate from the fluid to produce a sorbent. Preferably, the process further comprises washing the exchanged phyllosilicate after it is separated from the solution and/or washing the amended phyllosilicate after it is separated from the fluid and/or drying the amended phyllosilicate after it is washed and/or processing the solution separated from the exchanged phyllosilicate using a technique selected from the group consisting of reusing the solution, and treating the solution to recover unused metal ions. Preferably, the pyllosilicate substrate material is contacted with a solution containing a polyvalent metal ion selected from the group consisting of a bivalent tin ion, a tetravalent tin ion, a bivalent iron ion, a trivalent iron ion, a titanium ion, a manganese ion, a zirconium ion, a vanadium ion, a zinc ion, a nickel ion, a bismuth ion, a cobalt ion, and a molybdenum ion. Preferably, the exchanged phyllosilicate is separated from solution using settling, flotation, filtration, and centrifugation. Preferably, the phyllosilicate substitute material is contacted with the solution using consisting of batch contacting, co-current contacting, and/or counter-current contacting. Preferably, the exchanged phyllosilicate is contacted with a fluid selected from the group consisting of a gas phase source of sulfide, and a liquid phase source of sulfide. Preferably, the exchanged phyllosilicate is contacted with hydrogen sulfide gas. Preferably, the exchanged phyllosilicate is contacted with a solution containing a source of sulfide ions selected from the group consisting of sodium sulfide, sodium bisulfite, potassium sulfide, calcium sulfide, calcium polysulfide, ammonium sulfide, and another compound containing sulfur in the sulfide state. Preferably, the fluid is an aqueous solution and the process further comprises: adjusting the pH of the aqueous solution to a pH of in the range of 6 to 9 and more preferably, the pH is adjusted to within approximately plus or minus 0.5 pH units of pH 7.
In another preferred embodiment, the invention is a sorbent production system comprising: means for contacting a silicate substrate material with a solution containing a polyvalent metal ion other than a copper ion to produce an exchanged silicate; means for separating the exchanged silicate from the solution; means for contacting the exchanged silicate with a fluid containing a sulfide ion being devoid of a polysulfide ion to produce an amended silicate; means for separating the amended silicate from the fluid to produce a sorbent.
The invention is also a method and system for removing mercury from a gas stream. In a preferred embodiment, the invention is a method for removing mercury from a gas stream containing an acid gas, the method comprising: injecting and entraining a sorbent particle disclosed herein into the gas stream containing ionic and elemental mercury under conditions wherein at least a portion of said elemental and ionic mercury sorbs onto the collected sorbent particle during its exposure to the gas stream; and removing the sorbent particle from the gas stream. Preferably, the removing step is accomplished by means of a process selected from the group consisting of filtration, electrostatic precipitation, an inertial method, and wet scrubbing.
In a preferred embodiment, the invention is a method for removing mercury from a gas stream, the method comprising: injecting and entraining a sorbent particle disclosed herein into the gas stream containing ionic and elemental mercury under conditions wherein at least a portion of said elemental and ionic mercury sorbs onto the collected sorbent particle during its exposure to the gas stream; and removing the sorbent particle from the gas stream by means of a process selected from the group consisting of filtration, electrostatic precipitation, an inertial method, and wet scrubbing. Preferably, the injecting and entraining step involves injecting and entraining the sorbent particle into a flue gas stream containing a plurality acid gases including sulfur dioxide (SO2) in the range of a few hundred to a few thousand parts per million (ppm), hydrogen chloride (hydrochloric acid, HCl) up to 100 ppm, and nitrogen oxides (e.g., NO2) in the range of 200 to 2,000 ppm.
In a preferred embodiment, the invention is a process for removing mercury from a gas, the process comprising: contacting the gas containing mercury with a sorbent produced using a process disclosed herein.
In a preferred embodiment, the invention is a technique for removing mercury from a gas, the technique comprising: contacting a sorbent disclosed herein with a gas stream containing mercury at a temperature that does not exceed 350 degrees Fahrenheit for at least one second; removing the sorbent from the gas stream; and heating the sorbent to a temperature of about 500 degrees Fahrenheit to desorb the adsorbed mercury from the sorbent and produce a regenerated sorbent; and removing the adsorbed mercury from the vicinity of the regenerated sorbent.
In yet another preferred embodiment, the invention is a method for removing mercury from a gas, the method comprising: flowing the gas containing mercury through a fixed or fluidized bed comprised of a sorbent disclosed herein.
In a further preferred embodiment, the invention is a method for removing mercury from a gas, the method comprising: injecting and entraining a sorbent disclosed herein into a gas stream containing mercury at an operating pressure within about plus or minus 0.5 to 1.0 psig of ambient conditions; and removing the sorbent from the gas stream by filtration, electrostatic precipitation, inertial methods, and/or wet scrubbing. Preferably, at least a portion of said sorption occurs onto the collected sorbent while it remains exposed to the gas stream.
In a preferred embodiment, the invention is a system for removing mercury from a gas, the system comprising: means for flowing the gas containing mercury through a bed comprising of a sorbent described herein operating at gas temperatures greater than 500 degrees Fahrenheit and pressures greater than ambient conditions; and means for removing the mercury from the sorbent by reducing the operating pressure of the sorbent container, while maintaining the temperature of the sorbent at or near the normal operating temperature for the process.
In a preferred embodiment, the invention is a system for removing mercury from a gas, the system comprising: an injector for injecting a sorbent disclosed herein into a flue gas stream; a contactor for contacting the sorbent with the flue gas stream and producing a mercury-laden sorbent; and a separator for separating the mercury-laden sorbent from the flue gas stream. Preferably, the system further comprises: a regenerator for regenerating the mercury-laden sorbent.
In another preferred embodiment, the invention is a system for removing mercury from a flue gas, the system comprising: a source of flue gas that contains an acid gas (e.g., a power plant); means for exposing (e.g., an injection and entrainment system, a fixed bed or a fluidized bed) the flue gas to a sorbent disclosed herein. Preferably, if an injection and entrainment system is the selected means for exposing, the system also comprises means for separating the sorbent from the flue gas after the sorbent has contacted the flue gas for a time that is effective for the sorbent to capture mercury present in the flue gas.
Another preferred embodiment of the invention is a system for removing mercury from a gas, the system comprising: means for injecting a sorbent disclosed herein into a flue gas stream; means for contacting the sorbent with the flue gas stream and producing a mercury-laden sorbent; and means for separating the mercury-laden sorbent from the flue gas stream.
Another preferred embodiment of the invention is a method for removing mercury from a gas, the method comprising: a step for injecting a sorbent disclosed herein into a flue gas stream; a step for contacting the sorbent with the flue gas stream and producing a mercury-laden sorbent; and a step for separating the mercury-laden sorbent from the flue gas stream.
Another preferred embodiment of the invention is a facility that produces a flue gas that incorporates a system for removing mercury disclosed herein or a method of operating a facility that produces a flue gas in accordance with a method for removing mercury disclosed herein. In a preferred embodiment, the invention is a power plant comprising a system disclosed herein or a method for operating a power plant in accordance with a method disclosed herein. In another embodiment, the invention is a power grid energized at least in part by a power plant comprising a system for removing mercury disclosed herein. In another preferred embodiment, the invention is an incinerator comprising a system disclosed herein or a method for operating an incinerator in accordance with a method disclosed herein.
In another preferred embodiment, the invention is a concrete additive comprising a fly ash containing a sorbent disclosed herein that has been used to remove mercury from a gas stream and is mercury laden. In yet another embodiment, the invention is a method for making a concrete additive that comprises adding to a cement and aggregate mixture a fly ash containing a sorbent disclosed herein that has been used to remove mercury from a gas stream. In another embodiment, the invention is a concrete made by combining a cement, an aggregate and a fly ash containing a sorbent disclosed herein that has been used to remove mercury from a gas stream.